Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Fluid reactants are supplied to a pair of electrodes, which are in contact with and separated by an electrolyte. The electrolyte may be a solid or a liquid. In the solid polymer electrochemical fuel cells the electrodes typically comprise an electrode substrate and an electrocatalyst layer disposed upon one major surface of the electrode substrate. The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon fiber paper or carbon cloth. The layer of electrocatalyst is typically in the form of finely comminuted metal, typically platinum, and is disposed on the surface of the electrode substrate at the interface with the membrane electrolyte in order to induce the desired electrochemical reaction. In a single cell, the electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer.
The catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion-exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing gaseous fuel stream from the oxygen-containing gaseous oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product.
In liquid feed, electrochemical fuel cells, one or more of the reactants is introduced to the electrocatalyst in liquid form. Most commonly, methanol is the fuel supplied to the anode (so-called “direct methanol” fuel cells) and oxygen to the cathode. In fuel cells of this type, the reaction at the anode produces protons, which arise from the oxidation of methanol. An electrocatalyst promotes the methanol oxidation at the anode. The protons formed at the anode electrocatalyst migrate through the electrolyte from the anode to the cathode, and at the cathode electrocatalyst layer the oxidant reacts with the protons to form water.
In electrochemical fuel cells employing liquid or solid electrolytes and gaseous or liquid reactant streams, crossover of a reactant from one electrode to the other is generally undesirable. Reactant crossover may occur if the electrolyte is permeable to the reactant, that is, some of a reactant introduced at a first electrode of the fuel cell may pass through the electrolyte to the second electrode, instead of reacting at the first electrode. Reactant crossover typically causes a decrease in both reactant utilization efficiency and fuel cell performance. Fuel cell performance is defined as the voltage output from the cell at a given current density or vice versa; the higher the voltage at a given current density or the higher the current density at a given voltage, the better the performance.
In solid polymer, electrochemical fuel cells, the ion-exchange membrane may be permeable to one or more of the reactants. For example, ion-exchange membranes typically employed in solid polymer electrochemical fuel cells are permeable to methanol; thus, methanol which contacts the membrane prior to participating in the oxidation reaction can cross over to the cathode side. Diffusion of methanol fuel from the anode to the cathode leads to a reduction in fuel utilization efficiency and to performance losses (see, for example, S. Surampudi et al., Journal of Power Sources, Vol. 47, 377-385 (1994) and C. Pu et al., Journal of the Electrochemical Society, Vol. 142, L119-120 (1995)).
Fuel utilization efficiency losses arise from methanol diffusion away from the anode because some of the methanol which would otherwise participate in the oxidation reaction at the anode and supply electrons to do work through the external circuit is lost. Methanol arriving at the cathode may be lost through vaporization into the oxidant stream, or may be oxidized at the cathode electrocatalyst, consuming oxidant.
Methanol diffusion to the cathode may lead to a decrease in fuel cell performance. The oxidation of methanol at the cathode reduces the concentration of oxygen at the electrocatalyst and may affect access of the oxidant to the electrocatalyst. Further, depending upon the nature of the cathode electrocatalyst and the oxidant supply, the electrocatalyst may be poisoned by methanol oxidation products, or sintered by the methanol oxidation reaction. Diffusion of methanol across the H+-conducting porous polymer electrolyte membrane is one of the fundamental problems of the DMFC. To overcome it, it has been proposed to use barrier layers of electrolyte in the electrolyte membranes, particularly palladium layers.
It has also been proposed to provide improved polymer electrolyte membranes for DMFCs, utilizing cross-linked polystyrene sulfonic acid within electrochemically inert matrices of poly(vinylidene fluoride) or using other matrix membranes in place of polyvinylene fluoride or blended or co-polymerized with it.
U.S. Pat. No. 5,874,182 proposes a liquid feed electrochemical fuel cell comprising: a) a first electrode comprising a quantity of catalysts and a self-supporting porous sheet material having first and second appositely facing measured surfaces, said first electrode fluidly connected to a source of liquid reactant;    b) a second electrode;    c) an ion-exchange membrane interposed between said electrodes; wherein said catalyst is distributed through the thickness of said sheet material between said measured surfaces.
However, the solutions of the prior art are not fully satisfactory. Palladium is an expensive material and its use as a barrier layer involves lack of stability due to adhesion and oxidation problems. The membranes proposed in the prior art have lack of uniformity and decreased proton conductivity. At any rate, all the proposed modifications of the fuel cells do not result in significant reduction of methanol crossover, and the problem of methanol crossover is still significantly felt.
A general structure of fuel cells with which this invention is concerned is schematically indicated in FIG. 1. It comprises a cathode 10 and an anode 11 separated by a polymer electrolyte 12. Liquid fuel, in this case methanol, is supplied from a tank 13 to the anode and oxygen (or air) is supplied by a compressor 14 to the cathode. CO2 is discharged from tank 13, as indicated at 15. The polymer electrolyte is permeable to the methanol, which therefore can migrate through the polymer electrolyte to the cathode.
It is therefore a purpose of this invention to prevent methanol, and in general, liquid fuel, from coming into contact with the cathode, thereby poisoning the same.
It is another purpose of this invention to prevent the crossover of methanol without changing the polymer electrolyte.
It is a further purpose of this invention to provide a cathode provided with a methanol barrier that protects said cathode from being poisoned by the methanol.
It is a still further purpose of the invention to provide such an electrode whether its the surface is smooth or not.
It is a still further purpose to provide an improvement in fuel cell electrodes that is efficient even in H2/O2 fuel cells wherein the fuel is not methanol.
It is a still further purpose of the invention to permit to dispense, in certain cases, from the presence of the solid electrolyte.
It is a still further purpose of the invention to provide a process for producing said improved electrodes.
It is a still further purpose of the invention to achieve the aforesaid purposes without significantly hindering the oxygen reduction process if the cathode is an oxygen cathode.
It is a still further purpose of the invention to provide a process whereby commercially available catalytic electrodes, which can be used as oxygen cathodes in fuel cells, are improved to achieve the purposes of the invention.
Other purposes and advantages of the invention will appear as the description proceeds.